Coloured polyester films have numerous uses, for example as light filters, particularly in industrial applications, and in solar control applications such as window films for automobile, domestic and office windows. Polyester films are commonly coloured by dyeing with disperse dyes. A disperse dye may be defined generally as a substantially water-insoluble dye having substantivity for one or more hydrophobic polymers such as polyesters, for example poly(ethylene terephthalate).
One known method of dyeing a polyester film with a disperse dye relies upon swelling the film with an organic solvent. The dye is generally applied to the film in solution in the solvent, either at the same time as or subsequently to the swelling treatment. The former of these techniques is the one more commonly used. The dye diffuses into the swollen polyester film, which is then washed to remove the solvent and dried. The process of diffusion may be assisted by heating. This technique can be referred to as solvent dyeing. Solvent dyeing has the disadvantage that organic solvents capable of swelling polyester film and therefore suitable for the purpose are in general not environmentally friendly. The organic solvent may furthermore be difficult or expensive to remove from the film and to reclaim or to dispose of. The presence of residual solvent in the dyed film may affect subsequent processes such as metallisation.
Another known method of dyeing a polyester film utilises a suspension of disperse dye in a mixture of water and an organic solvent which swells the film. This method may be called solvent-assisted dyeing, and is otherwise similar to solvent dyeing.
Another known method of dyeing a polyester film with a disperse dye relies upon thermal diffusion of the dye into the film. The dye is brought into contact with the film which is then heated to cause migration of the dye into the film. The dye may be coated onto the film and the coated film dried and heated, or the dye may be provided on a separate sheet which is brought into contact with the film and the sheet and film are then heated. The former process may be referred to as theremofixation or thermosol fixation dyeing and the latter process as transfer printing. The heating step is generally carried out at or around the sublimation temperature of the dye. The process of transfer of dye into the film may be referred to as fixation.